Biosensor systems provide an analysis of a biological fluid, such as whole blood, serum, plasma, urine, saliva, interstitial, or intracellular fluid. Typically, biosensor systems have a measurement device that analyzes a sample contacting a sensor strip. The sample is typically in liquid form and in addition to being a biological fluid, may be the derivative of a biological fluid, such as an extract, a dilution, a filtrate, or a reconstituted precipitate. The analysis performed by the system determines the presence and/or concentration of one or more analytes, such as alcohol, glucose, uric acid, lactate, cholesterol, bilirubin, free fatty acids, triglycerides, proteins, ketones, phenylalanine, or enzymes, in the biological fluid. The analysis may be useful in the diagnosis and treatment of physiological abnormalities. For example, a diabetic individual may use a biosensor system to determine the glucose level in whole blood for adjustments to diet and/or medication.
Biosensor systems may be designed to analyze one or more analytes in the same or in different samples and may use different sample volumes. Some systems may analyze a single drop of whole blood, such as from 0.25-15 microliters (μL) in volume. Biosensor systems may be implemented using bench-top, portable, and like measurement devices. Portable measurement devices may be hand-held and allow for the identification and/or quantification of one or more analytes in a sample. Examples of portable measurement devices include the Breeze II® and Contour® meters of Bayer HealthCare Diabetes Care in Tarrytown, N.Y., while examples of bench-top measurement devices include the Electrochemical Workstation available from CH Instruments in Austin, Tex. Systems providing shorter analysis times, while supplying the desired accuracy and/or precision, provide a substantial benefit to the user.
In electrochemical biosensor systems, the analyte concentration is determined from an electrical signal generated by an oxidation/reduction or redox reaction of a measurable species. The measurable species may be ionized analyte or an ionized species responsive to the analyte when an input signal is applied to the sample. The input signal may be applied as a single pulse or in multiple pulses, sequences, or cycles. An oxidoreductase, such as an enzyme or similar species, may be added to the sample to enhance the electron transfer from a first species to a second species during the redox reaction. The enzyme or similar species may react with a single analyte, thus providing specificity to a portion of the generated output signal. Examples of some specific oxidoreductases and corresponding analytes are given below in Table I.
TABLE IOxidoreductaseAnalyteGlucose dehydrogenaseβ-glucoseGlucose oxidaseβ-glucoseCholesterol esterase; cholesterol oxidaseCholesterolLipoprotein lipase; glycerol kinase; glycerol-Triglycerides3-phosphate oxidaseLactate oxidase; lactate dehydrogenase;LactatediaphorasePyruvate oxidasePyruvateAlcohol oxidaseAlcoholBilirubin oxidaseBilirubinUricaseUric acidGlutathione reductaseNAD(P)HCarbon monoxide oxidoreductaseCarbon monoxide
A mediator may be used to maintain the oxidation state of the enzyme. In maintaining the oxidation state of the enzyme, the mediator is ionized and may serve as a measurable species responsive to the analyte. Table II, below, provides some conventional combinations of enzymes and mediators for use with specific analytes.
TABLE IIAnalyteEnzymeMediatorGlucoseGlucose OxidaseFerricyanideGlucoseGlucose DehydrogenaseFerricyanideCholesterolCholesterol OxidaseFerricyanideLactateLactate OxidaseFerricyanideUric AcidUricaseFerricyanideAlcoholAlcohol OxidasePhenylenediamine
The mediator may be a one electron transfer mediator or a multi-electron transfer mediator. One electron transfer mediators are chemical moieties capable of taking on one additional electron during the conditions of the electrochemical reaction. One electron transfer mediators include compounds, such as 1,1′-dimethyl ferrocene, ferrocyanide and ferricyanide, and ruthenium(III) and ruthenium(II) hexaamine. Multi-electron transfer mediators are chemical moieties capable of taking on more-than-one electron during the conditions of the reaction. Multi-electron transfer mediators include two electron transfer mediators, such as the organic quinones and hydroquinones, including phenanthroline quinone; phenothiazine and phenoxazine derivatives; 3-(phenylamino)-3H-phenoxazines; phenothiazines; and 7-hydroxy-9,9-dimethyl-9H-acridin-2-one and its derivatives. Two electron transfer mediators also include the electro-active organic molecules described in U.S. Pat. Nos. 5,393,615; 5,498,542; and 5,520,786.
Two electron transfer mediators include 3-phenylimino-3H-phenothiazines (PIPT) and 3-phenylimino-3H-phenoxazines (PIPO). Two electron mediators also include the carboxylic acid or salt, such as ammonium salts, of phenothiazine derivatives. Two electron mediators further include (E)-2-(3H-phenothiazine-3-ylideneamino)benzene-1,4-disulfonic acid (Structure I), (E)-5-(3H-phenothiazine-3-ylideneamino)isophthalic acid (Structure II), ammonium (E)-3-(3H-phenothiazine-3-ylideneamino)-5-carboxybenzoate (Structure III), and combinations thereof. The structural formulas of these mediators are presented below. While only the di-acid form of the Structure I mediator is shown, mono- and di-alkali metal salts of the acid are included. The sodium salt of the acid may be used for the Structure I mediator. Alkali-metal salts of the Structure II mediator also may be used.

Two electron mediators may have redox potentials that are at least 100 mV lower, more preferably at least 150 mV lower, than ferricyanide. Other two electron mediators may be used.
Electrochemical biosensor systems typically include a measurement device having electrical contacts that connect with electrical conductors in the sensor strip. The sensor strip may be adapted for use outside, in contact with, inside, or partially inside a living organism. When used outside a living organism, a sample of the biological fluid may be introduced to a sample reservoir of the sensor strip. The sensor strip may be placed in the measurement device before, after, or during the introduction of the sample for analysis. When in contact with the living organism, the sensor strip may be attached to the skin where fluid communication is established between the organism and the strip. When inside or partially inside a living organism, the sensor strip may be continually immersed in the fluid or the fluid may be intermittently introduced to the strip for analysis. The sensor strip may include a reservoir that partially isolates a volume of the fluid or be open to the fluid. When in contact with, partially inside, or inside a living organism, the measurement device may communicate with the sensor strip using wires or wirelessly, such as by RF, light-based, magnetic, or other communication techniques.
The conductors of the sensor strip may be made from conductive materials, such as solid metals, metal pastes, conductive carbon, conductive carbon pastes, conductive polymers, and the like. The electrical conductors typically connect to working, counter, reference, and/or other electrodes that extend into a sample reservoir. One or more electrical conductors also may extend into the sample reservoir to provide functionality not provided by the electrodes.
The measurement device applies an input signal to the electrical conductors of the sensor strip. The electrical conductors convey the input signal through the electrodes into the sample. The redox reaction of the measurable species generates an electrical output signal in response to the input signal. The electrical output signal from the strip may be a current (as generated by amperometry or voltammetry), a potential (as generated by potentiometry/galvanometry), or an accumulated charge (as generated by coulometry). The measurement device may have the processing capability to measure and correlate the output signal with the presence and/or concentration of one or more analytes in the biological fluid. The processing capability may be in communication with the measurement device, but separate. Communication may be established using wires or wirelessly, such as by RF, light-based, magnetic, or other communication.
In coulometry, the analyte concentration is quantified by exhaustively oxidizing the analyte within a small volume and integrating the current over the time of oxidation to produce the electrical charge representing the analyte concentration. Thus, coulometry captures the total amount of analyte within the sensor strip. An important aspect of coulometry is that towards the end of the integration curve of charge vs. time, the rate at which the charge changes with time becomes substantially constant to yield a steady-state condition. This steady-state portion of the coulometric curve forms a relatively flat current region, thus allowing determination of the corresponding current. However, the coulometric method requires the complete conversion of the entire volume of analyte to reach the steady-state condition unless the true steady-state current is estimated from non-steady-state output. As a result, this method may be time consuming or less accurate due to the estimation. The sample volume of the sensor strip also must be controlled to provide accurate results, which can be difficult with a mass produced device.
Another electrochemical method which has been used to quantify analytes in biological fluids is amperometry. In amperometry, current is measured at a substantially constant potential (voltage) as a function of time as a substantially constant potential is applied across the working and counter electrodes of the sensor strip. The measured output current is used to quantify the analyte in the sample. Amperometry measures the rate at which the electrochemically active species, such as the analyte or mediator, is being oxidized or reduced near the working electrode. Many variations of the amperometric method for biosensors have been described, for example in U.S. Pat. Nos. 5,620,579; 5,653,863; 6,153,069; and 6,413,411.
Voltammetry is another electrochemical method that may be used to quantify analytes in biological fluids. Voltammetry differs from amperometry in that the potential of the input signal applied across the working and counter electrodes of the strip changes continuously with time. The current is measured as a function of the change in potential of the input signal and/or time. Additional information about voltammetry may be found in “Electrochemical Methods: Fundamentals and Applications” by A. J. Bard and L. R. Faulkner, 1980.
Multiple methods of applying the input signal to the strip, commonly referred to as pulse methods, sequences, or cycles, have been used to address inaccuracies in the determined analyte concentration. For example, in U.S. Pat. No. 4,897,162 the input signal includes a continuous application of rising and falling voltage potentials that are commingled to give a triangular-shaped wave. Furthermore, WO 2004/053476 and U.S. Patent Docs. 2003/0178322 and 2003/0113933 describe input signals that include the continuous application of rising and falling voltage potentials that also change polarity.
Electrochemical decays may be correlated with the analyte concentration in the sample by expressing the decay with an equation describing a line relating current with time by the natural log function (In), for example. Thus, the output current may be expressed as a function of time with an exponential coefficient, where negative exponential coefficients indicate a decay process. After the initial decrease in current output, the rate of decrease may remain relatively constant, thus becoming steady-state, or continue to fluctuate.
The measurement performance of a biosensor system is defined in terms of accuracy and/or precision. Increases in accuracy and/or precision provide for an increase in measurement performance for the biosensor system. Accuracy may be expressed in terms of bias of the sensor's analyte reading in comparison to a reference analyte reading, with larger bias values representing less accuracy, while precision may be expressed in terms of the spread or variance among multiple analyte readings in relation to a mean. Bias is the difference between a value determined from the biosensor and the accepted reference value and may be expressed in terms of “absolute bias” or “relative bias”. Absolute bias may be expressed in the units of the measurement, such as mg/dL, while relative bias may be expressed as a percentage of the absolute bias value over the reference value. Reference values may be obtained with a reference instrument, such as the YSI 2300 STAT PLUS™ available from YSI Inc., Yellow Springs, Ohio.
Many biosensor systems include one or more methods to correct the error, and thus the bias, associated with an analysis. The concentration values obtained from an analysis with an error may be inaccurate. The ability to correct these inaccurate analyses may increase the accuracy and/or precision of the concentration values obtained. An error correction system may compensate for one or more errors, such as error arising when the measurable species concentration does not correlate with the analyte concentration. For example, when a biosensor system determines the concentration of a reduced mediator generated in response to the oxidation of an analyte, any reduced mediator not generated by oxidation of the analyte will lead to the system indicating that more analyte is present in the sample than is correct due to mediator background. Thus, “mediator background” is the bias introduced into the measured analyte concentration attributable to measurable species not responsive to the underlying analyte concentration.
Measurement inaccuracies also may arise when the output signal does not correlate to the measurable species concentration of the sample. For example, when a biosensor system determines the concentration of a measurable species from output signal currents, output currents not responsive to the measurable species will lead to the system indicating that more analyte is present in the sample than is correct due to interferent current. Thus, “interferent bias” is the bias introduced into the measured analyte concentration attributable to interferents producing output currents not responsive to the underlying analyte concentration.
As may be seen from the above description, there is an ongoing need for electrochemical sensor systems having improved measurement performance, especially those that may provide an increasingly accurate and/or precise determination of a biological analyte concentration. The systems, devices, and methods of the present invention overcome at least one of the disadvantages associated with conventional systems.